1. Field of the Invention
This invention relates to an apparatus and method for extinguishing fires consuming combustible fluids issuing from conduits such as pipes, gas wells and vent stacks. These combustible fluids may be liquids or gases.
There are a variety of situations wherein a combustible fluid is conducted from a source through a pipe to an outlet of the pipe where there is the possibility of ignition of the combustible fluid and ensuing fire as the combustible fluid continues to flow. Notable examples are vent stacks communicating from the ullage space on tanks containing combustible fluids to a vapor disposal area. Specific examples are the vent stacks on mobile tank transports for both cryogenically liquefied flammable gases and normally liquid combustible fluids having a high vapor pressure, such as gasoline and propane.
A particularly acute problem occurs in the venting of gaseous hydrogen such as from the boiloff of a liquid hydrogen storage or transport tank. The mixture of venting hydrogen and atmospheric oxygen produces a mixture having an extremely low ignition energy level. It is not uncommon that hydrogen venting to the atmosphere from a vent stack will spontaneously ignite, resulting in a very high temperature, virtually invisible flame. The low ignition energy, the high diffusivity and flame front velocities of a hydrogen and oxygen mixture combine to make extinguishment of such a fire very difficult. Typically, neither cooling nor diluting the mixture will extinguish the flame. It is also often difficult to eliminate the supply of oxygen by blanketing at the exit of the vent stack with inert gases. Thus, to extinguish such a fire it is necessary to cut off the flow of fuel at least for a short period of time
2. Description of the Prior Art
The prior art has generally relied upon mechanical means for blocking the flow of combustible fluid through the pipe to the exit point where it is burning. These mechanical means have included various types of block valves or, in the case of oil and gas wells, certain types of blowout preventers. Problems associated with these mechanical blocking means include speed of operation and reliability. A further problem is that the blocking action of the mechanical device essentially stops the flow of the combustible fluid downstream from the blocking device unless an additional purge is introduced. This can have the effect of allowing the flame front to move into the conduit or allowing pockets of combustible mixtures to remain in the conduit downstream of the block valve for extended periods of time. This can create an unsafe situation. Mechanical block valves may also jam closed at cryogenic temperatures or at high temperatures caused by the fire. This jamming may cause serious pressure buildup in the storage space after the fire has been successfully extinguished.
U.S. Pat. No. 1,640,839 to Kliewer discloses a fire extinguisher for oil wells which relies not on mechanical blocking but upon interruption of the combustible fluid flow by means of flat blankets or jets of large volumes of steam directed from circumferential slots across the flow passage. The introduction of a large volume of steam chokes off and dilutes the flow of gas or oil. According to the Kliewer teahings, large volumes of steam must be introduced to choke off the fuel flow. These large volumes may not be critical where steam in large volumes is readily available, but presents a problem either where such volumes are not available or in those instances in which an expensive extinguisher fluid is required, such as helium.